Washington, May 2 : Astronomers have confirmed the existence of a new type of pulsating white dwarf star, which is about 800 light-years away in the constellation Ursa Major.

The star was discovered by University of Texas at Austin astronomers Michael H. Montgomery and Kurtis A. Williams, along with student Steven DeGennaro, with the help of the 2.1-meter Otto Struve Telescope at McDonald Observatory.

A white dwarf star is the leftover remnant of a Sun-like star that has burned all of the nuclear fuel in its core. It is extremely dense, packing half to 1.5 times the Sun's mass into a volume about the size of Earth.

Until recently, there have been two main types of white dwarfs known: those that have an outer layer of hydrogen (about 80 percent), and about those with an outer layer of helium (about 20 percent), whose hydrogen shells have somehow been stripped away.

Last year, University of Arizona astronomers Patrick Dufour and James Liebert discovered a third type of white dwarf star, still more rare.

For reasons that are not understood, these "hot carbon white dwarfs" have had both their hydrogen and helium shells stripped off, leaving their carbon layer exposed.

Astronomers suspect these could be among the most massive white dwarfs of all, and are the remnants of stars slightly too small to end their lives in a supernova explosion.

After these new carbon white dwarfs were announced, Montgomery calculated that pulsations in these stars were possible.

Pulsating stars are of interest to astronomers because the changes in their light output can reveal what goes on in their interiors - similar to the way geologists study seismic waves from earthquakes to understand what goes on in Earth's interior.

So, Montgomery and Williams' team began a systematic study of carbon white dwarfs with the Struve Telescope at McDonald Observatory, looking for pulsators.

DeGennaro discovered that a star about 800 light-years away in the constellation Ursa Major, called SDSS J142625.71+575218.3, fits the bill. Its light intensity varies regularly by nearly two percent about every eight minutes.

Called a "pulsating carbon white dwarf," this is the first new class of variable white dwarf star discovered in more than 25 years.

"The discovery that one of these stars is pulsating is remarkably important," said National Science Foundation astronomer Michael Briley.

"This will allow us to probe the white dwarf's interior, which in turn should help us solve the riddle of where the carbon white dwarfs come from and what happens to their hydrogen and helium," he added.